Project Summary The long term goals of this research involve modification of the peptidyltransferase (PTC) center in bacterial ribosomes to enable the incorporation of very unusual amino acids not recognized by wild- type ribosomes. In recent years, we have described the selection of modified ribosomes and their use in incorporating D-amino acids, beta amino acids, dipeptides and dipeptidomimetric cassettes into proteins. During the five years of proposed research, the focus of efforts will be on the introduction of conformationally constrained cyclic dipeptides and nucleobase amino acids into specific proteins (both as mono- and dipeptides), creating species that have novel properties enabling enhanced analysis of protein function to be realized, as well as the analysis of specific conformational properties essential for enzyme function and strategies for enhancing (and potentially modifying) protein-DNA recognition.